Not sure what it is but don't stand too close as it opens up!

It looks like the FEED ME plant from Little Shop of Horrors movie. I'd love to see it bloom. Where are you located? That will help ID it maybe.

On a hunch it may be related to a poppy flower bud, looks similar in googled pics.

Got a lot of that weed in my garden too. It appears to be a wild mallow, there are 2 other mallow species growing in my garden as well.

Looks like a thistle to me. Pull it and use gloves because it has tiny thorns along the stem.

Thank you so much everyone! Special shout out to @donna_in_sask! Google images of Hibiscus trionum matche what's growing! I'm going to let it stick around!

Thank you, Donna... at least now I know what to call it. ;-) It is heavily established in my garden, and blooms (and seeds) very rapidly. I intentionally introduced Malva sylvestris into my garden as a food source for pollinators, the two species now self-seed in about equal numbers. Both supply pollen in large quantities over a long period, and have strong tap roots that penetrate the subsoil.